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Hydrogen production using mixed oxides: TiO2-M (CoO and W[O.sub.3]).

1. Introduction

An important process for future energy supplies turns to be "photohydrogen" production from water splitting; the photocatalytic processes is clean and employ a renewable source as it has been reported for various systems [1-3]. Titanium dioxide (Ti[O.sub.2]) is considered as the best photocatalysts for dyes degradation [4-9] pesticides [10-14] atmospheric pollutants [15] as well as for the inorganic pollutant removal from wastewater [16,17]. However, the use of Ti[O.sub.2] as photocatalysts for water splitting is limited by its redox potential referring to the normal hydrogen electrode (NHE). Important studies have been made to improve the photocatalytic activity of titanium dioxide for the water splitting reaction. In this way, modified titanium dioxide prepared by doping with Fe, Zn, Cu, Ni, V, Mg, Be and Ni [18,19], or by impregnating the Ti[O.sub.2] with noble metals Pt, Pd, Ir, Rh, Ru [20]. In particular, the preparation of mixed oxides like CuO, ZnO, NiO and CeO [21-24], has attracted attention for researchers because they are low cost materials showing important photocatalytic properties. The incorporated oxide effect has been related to oxygen vacancies in its crystal structure [25,26]. With the propose to obtain improved titania photocatalysts with high hydrogen production from water splitting, in the present work Ti[O.sub.2]-CoO and Ti[O.sub.2]-W[O.sub.3] mixed oxides were prepared by the sol-gel method. Cobalt oxide and tungsten trioxide has been chosen as co-participant oxide because its energetic conduction and valence bands positions are in the favorable redox region for the water splitting [27], additionally it reduces the recombination rate of photogenerated [e.sup.-]-[h.sup.+] pairs [28]. The characterization of the catalysts has been done by nitrogen adsorption, XRD, Raman and UV-Vis spectroscopies. The water splitting reaction has been carried out at room temperature using a water-ethanol solution irradiated with a high pressure of mercury lamp. Ethanol reacts with the photogenerated holes which is an irreversible reaction performed on the semiconductor surface and the photocatalytic activity toward reduction of water to produce hydrogen is improved [29].

2. Experimental

2.1 Catalyst preparation

The nanostructured Ti[O.sub.2]-CoO and Ti[O.sub.2]-W[O.sub.3] samples were prepared by the sol-gel method using titanium (IV) butoxide (Aldrich 97%), cobalt nitrate (Reasol 99%) and tungstic acid (Fluka 99%) as precursors: 44 mL of 1-butanol (Aldrich 99.4%) and 18 mL of distilled water containing the appropriated amount of Co(N[O.sub.3]).6[H.sub.2]O to obtain solids with 1.0, 3.0, and 5.0 wt.%, were mixed and a few drops of HN[O.sub.3] were added in order to obtain pH=3 in the solution. After the solution was heated under reflux at 70[degrees]C and then 44 mL of titanium (IV) butoxide were added drop wise (water/alkoxide molar ratio 8) and maintaining during 4 h under magnetic stirring until the gel was formed. Afterwards, the gel was dried at 70[degrees]C during 24 h and the solid was ground to a fine powder in an agate mortar. The obtained xerogel was then annealed at 500[degrees]C during 5 h in static air atmosphere using a heating rate of 1[degrees]C/min; finally the product was ground again. As reference pure Ti[O.sub.2] sample was prepared in the same way described above but with no adding the Co and W precursor.

2.2 Catalyst characterization

2.2.1 Physisorption analysis

Nitrogen adsorption-desorption isotherms were obtained with an automatic Quantachrome Autosorb 3B instrument. Prior to the nitrogen adsorption, all the samples were outgassed overnight at 200[degrees]C. The specific surface areas of the samples were calculated from the nitrogen adsorption-desorption isotherms using the BET method, and the mean pore size diameter form the desorption isotherms using the BJH method.

2.2.2 X-ray powder diffraction

The obtained powders Ti[O.sub.2] and doped Ti[O.sub.2]-ZnO were analyzed by X-ray diffraction using a Bruker D-8 Advance apparatus. The diffraction intensity as a function of the diffraction angle (2[theta]) was measured between 4 and 70[degrees], using a step of 0.03[degrees] and a counting time of 0.3 s per step.

2.2.3 Raman Spectroscpy

Los espectros Raman fueron obtenidos en un espectrometro marca Renishaw, modelo MicroRaman Invia, utilizando un objetivo de 100X y como fuente de radiacion monocromatica un laser de Argon, con longitud de onda de emision de 514.5 nm correspondiente a la luz verde, y una potencia de salida de 25 mW. En el equipo de analisis fueron colocados 10 mg de muestra en polvo de los solidos. El intervalo de desplazamiento Raman para el analisis fue de 0 a 1200 [cm.sup.-1].

2.2.4 Infrared FT-IR

The FT-IR studies were realized by IR-Shimadzu equipped with ATR, with a wavelength de 500 [cm.sup.-1] to 4000 [cm.sup.-1].

2.2.5 UV-Vis diffuse reflectance spectroscopy

The UV--Vis absorption spectra were obtained with a Varian Cary 100 UV--Vis spectrophotometer coupled with an integration sphere for diffuse reflectance studies. A sample of MgO with a 100% reflectance was used as a reference. The diffuse reflectance spectrum was obtained and transformed to a magnitude proportional to the extinction coefficient ([alfa]) through the Kubelka-Munk function, Equation 1.

F(R) = [(1-R).sup.2]/2R (1)

The Eg was then calculated from the plot of the modified Kubelka-Munk function F(R) vs wave-lenght of the absorbed light.

2.2.6 Photocatalytic [H.sub.2] production

The photoactivity for the hydrogen generation was evaluated using a homemade Pyrex reactor of 200 mL containing an aqueous solution water-ethanol (1:1 molar ratio) and 0.1 g of catalysts. The photoirradiation was made using a high pressure Hg pen-lamp (with a radiation of 254 nm and intensity of 2.2 mW/[cm.sup.2]) encapsulated in a quartz tube immersed in the water solution. The amount of hydrogen produced was followed by gas chromatography using a Varian CP-3800 gas chromatograph equipped with a thermal conductivity detector and with a molecular sieve 5A column (30m length, 0.35mm ID and 50 mm OD).

3. Results and discussion

3.1 Specific surface area

The specific surface areas of the samples annealed at 500[degrees]C are reported in Table 1. The results show that specific surface area by the BET method of the Ti[O.sub.2]-CoO and Ti[O.sub.2]-W[O.sub.3] semiconductors is higher than obtained with the bare Ti[O.sub.2]. As the amount of Co and W increases an increment in the specific surface area was observed. It increments from 90 to 95 [m.sup.2]/g for the Ti[O.sub.2]-CoO and 48 to 91 [m.sup.2]/g for the Ti[O.sub.2]-W[O.sub.3] solids respectively. This result suggest that the pores of Ti[O.sub.2]-CoO mixed oxide are cylinders perfect and present the same pore size, with Ti[O.sub.2]-W[O.sub.3] the result suggest that, it present an increase of width pore when the wt.% of W increase.

3.2 X-ray diffraction

Figure 1 shows the X-ray diffraction patterns of the Ti[O.sub.2] and 1.0, 3.0 and 5.0% cobalt oxide and tungsten oxide with Ti[O.sub.2] samples. The nanocrystalline anatase structure was confirmed by (1 0 1), (0 0 4), (2 0 0), (1 0 5) and (2 1 1) diffraction peaks [30,33]. The XRD patterns of anatase have a main peak at 2[theta] = 25.2 corresponding to the 101 plane while the main peaks of rutile and brookite phases are at 2[theta] = 27.4 (110 plane) and 2[theta] = 30.8 (121 plane), respectively. Therefore, rutile and brookite phases have not been detected [31,32]. The XRD patterns didn't show any Co and W phase (even for 5% CoO-Ti[O.sub.2] and W[O.sub.3]-Ti[O.sub.2] sample) indicating that Co and W ions are uniformly dispersed among the anatase crystallites [34,35]. In the region of 2[theta] = 10-80, the shape of diffractive peaks of the crystal planes of pure Ti[O.sub.2] (sample a) is quite similar to that of CoO-Ti[O.sub.2] and W[O.sub.3]-Ti[O.sub.2] (samples b to g) with different concentrations of Co and W. The average particle size was estimated from the Scherrer equation on the anatase (2[theta] = 25.2, 37.8, and 48.1) diffraction peaks (the most intense peaks for each sample): D = K[lambda]/([beta] cos [theta]).

where D is the crystal size of the catalyst, [lambda] the X-ray wavelength (1.54056 [Angstrom]), [beta] the full width at half maximum of the diffraction peak (radian), Ka is a coefficient (0.89) and [theta] is the diffraction angle at the peak maximum. Average crystal sizes of Ti[O.sub.2] and CoO-Ti[O.sub.2] were calculated to be around 12-14 nm and 11-13 nm, respectively.

[FIGURE 1 OMITTED]

3.3 Raman spectroscopy

Raman spectra of all samples are represented in Figure 2 and were compared with those reported in the literatura [36,38]. You can appreciate four Raman peaks of anatase phase which are to 145.8 [cm.sup.-1], 397.9 [cm.sup.-1], 513 [cm.sup.-1] y 640.7 [cm.sup.-1]; which are assigned to modes 2B1g and the mode 2Eg respectively. Also presented are confirmed in X-ray, as there is a decrease of the peaks increase as the content of CoO and W[O.sub.3].

3.3 FT-IR spectroscopy

FT-IR spectra of Ti[O.sub.2] and 1.0, 3.0 and 5.0 wt.% Co and W samples (Figure 3) show peaks corresponding to stretching vibrations of the O-H and bending vibrations of the adsorbed water molecules. The intensity of these peaks in 3350-3450 cm-1 is lower because the annealing temperature which indicates the removal of a large portion of the adsorbed water from Ti[O.sub.2] (not shown in the figure) [39]. The broad intense band below 1200 cm-1 is due to Ti-O-Ti vibrations. The shift to the lower wavenumbers and sharpening of the Ti-O-Ti band from "b" to "g" in Figure 3 may be due to decrease in size of the catalyst nanoparticles with increasing Co and W in the formation of mixed oxide with the Ti[O.sub.2] from 1% to 5.0%, respectively. In addition, the surface hydroxyl groups in Ti[O.sub.2] increase with the increase of Co and W wt.%, which is confirmed by increase in intensity of the corresponding peaks. There is no band centered at 1389 [cm.sup.-1] due to the bending vibrations of the C-H bond in the catalysts of the mixed oxide [39]. Also, there are no excess bands assigned for the alkoxy groups.

[FIGURE 2 OMITTED]

3.4 UV-Vis Diffuse reflectance spectroscopy (DRS)

The electronic bands of the different titania samples were studied whose corresponding spectra are provided in Figure 3. The absorption spectrum of Ti[O.sub.2] consists of a single broad intense absorption around 400 nm due to the charge-transfer from the valence band (mainly formed by 2p orbitals of the oxide anions) to the conduction band (mainly formed by 3d t2g orbitals of the [Ti.sup.4+] cations) [40]. This absorption is similar to Ti[O.sub.2]-W[O.sub.3] mixed oxide because present a spectrum to 400 nm. The Ti[O.sub.2] and W[O.sub.3]-Ti[O.sub.2] mixed oxide showed absorbance in the shorter wavelength region while CoO-Ti[O.sub.2] and the DRS results showed a red shift in the absorption onset value in the case of CoO added titania. The mixed oxide of various transitional metal ions into Ti[O.sub.2] could shift its optical absorption edge from UV into visible light range, but no prominent change in Ti[O.sub.2] band gap was observed [34].

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Figure 5 shows the spectroscopy of Ti[O.sub.2]-CoO samples before of reaction, where the base line was realized with Ti[O.sub.2] and it indicates that its absorbance is 750 nm with the Ti[O.sub.2]-CoO all the samples, when increases CoO content there is a displacement to 372 nm for the Ti[O.sub.2]-CoO with 3.0 and 5.0 wt% and this form we verified that CoO is disperse in the titania lattice. The same procedure for the W[O.sub.3] was realized, see Figure 5, where it show two peaks in 362 nm with 1.0 wt%, when increase W[O.sub.3] content there is a displacement to 369 nm and 375 nm for the samples with content 3.0 and 5.0 wt% respectively. The second peaks are in the region of 400 nm to 450 nm that suggest a formation of Plasmon.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

The Figure 7 spectra, was obtained on powders after reaction. The analysis shows that at 225 nm probably exist a transition of the catalyst of [Ti.sup.4+] to [Ti.sup.3+], but in the region of 362 to 375 nm where W[O.sub.3] is present, only Co with 1.0 wt% present a little change (sample e), the peaks after of reaction diminished that confirm change in the material of Ti[O.sub.2]-W[O.sub.3] samples. With respect to Ti[O.sub.2]-CoO samples this present a little change in the region of 544 nm, which suggest that there are a change [Co.sup.2+] to [Co.sup.3+]. In the region of 400 to 500 nm present change significatives that suggest the Ti[O.sub.2]-CoO samples aren't changes.

[FIGURE 7 OMITTED]

Figure 8 shows the hydrogen production as a function of the irradiation time for bare Ti[O.sub.2], Ti[O.sub.2]-CoO and Ti[O.sub.2]-W[O.sub.3]. It can be seen that the hydrogen formation increases with the Co and W wt. %. The hydrogen production from bare Ti[O.sub.2] was 190 [micro]mol/h. An important effect of cobalt oxide is observed, the H2 formation for Ti[O.sub.2] CoO and Ti[O.sub.2]-W[O.sub.3] with 5 wt% was 900 and 1000 [micro]mol/h, respectively, an increase of 500 percent approximately. In this case of the above 5 wt. % CoO and W[O.sub.3] is the key to improved hydrogen photocatalytic production. This results are very interesting compared when is used titania nanotubes with Ir and Co [41] nanocomposites of CoO [42], Pt loaded Ti[O.sub.2] [43] W[O.sub.3]-Ti[O.sub.2] [44].

[FIGURE 8 OMITTED]

4. Conclusions

Ti[O.sub.2], Ti[O.sub.2]-CoO and Ti[O.sub.2]-W[O.sub.3] nanoparticles were prepared by the sol-gel method. From among all of the samples only anatase phase was confirmed from the XRD results. From the XRD, UV-Vis and FT-IR results, it was confirmed that the incorporation of Co and W in Ti[O.sub.2] decreases the grain size, shifts the absorption to higher wavelenghts (red shift) and lowers the surface area due to agglomeration of the particles. The photocatalytic activity of hydrogen production under UV irradiation revealed higher activity in the presence of the mixed oxide. Among the Co and W samples, the catalyst exhibited the highest photocatalytic activity, while under visible irradiation, the best catalyst was the 5.0% Ti[O.sub.2]-CoO.

Fecha de recepcion del articulo: 24/06/2013 Fecha de aceptacion del articulo: 10/07/2013

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Alejandro Perez-Larios (1,2) *; Ricardo Gomez (1).

(1) Universidad Autonoma Metropolitana-Iztapalapa, Depto. de Quimica, Area de Catalisis, Grupo ECOCATAL, Av. San Rafael Atlixco No 189, Mexico 09340, D.F. Mexico. *alex.perez.larios@gmail.com

(2) Universidad de Guadalajara, Centro Universitario de Tonala, Division de Ingenierias, Sede Provisional Casa de laCultura--Administracion: Morelos # 180, Zona Centro, Tonala, Jalisco, Mexico. 45400
Table 1. Textural properties, band gap and [H.sub.2] production

                                     [H.sub.2]
Catalysts        Area        Eg      Production     Crystal
(wt%)         [m.sup.2]/g   (eV)   ([micro]mol/h)    Size D

Co 1.0            90        1.59        562            12
Co 3.0            93        1.75        864            13
Co 5.0            95        2.44        1008           14
W 1.0             48        3.02        600            11
W 3.0             53        3.12        884            12
W 5.0             91        3.08        956            13
Ti[O.sub.2]       64        3.2         190           7.2
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Date:Jul 1, 2013
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